Vehicle steering systems having more than one set of steerable wheels are known. Typically, such a steering system includes a front set of steerable wheels and a rear set of steerable wheels, both of which are controlled in response to turning of the vehicle steering wheel. A steering system having more than one set of steerable wheels has particular suitability in a large vehicle such as a truck so as to increase steering maneuverability.
U.S. Pat. No. 4,175,638 discloses a vehicle steering system having steerable front wheels and steerable rear wheels. Turning of the front wheels and the rear wheels is controlled in response to turning of a steering wheel and position of a steering mode selector lever. The selector lever can be positioned so as to select one of a plurality of steering modes. These steering modes include (i) front wheel only steering, (ii) crab steering, (iii) curl steering (also referred to as coordinated or radiarc steering), and (iv) rear wheel only steering. During operation of the vehicle, the selector lever can be moved to effect a change in steering mode. The '638 patent further discloses a lock mechanism which, upon actuation, locks the selector lever in its present position. Although the disclosed arrangement locks the selector lever when the engine is off, it is mentioned that the lock mechanism could also be used to lock the lever in a given position while the vehicle is in operation. The '638 patent further discloses, in a second embodiment, a safety mechanism which prevents moving of a control member between more than two positions without cautioning the operator that such action is taking place.
In such a steering system, steering mode changes are made independent of vehicle speed. As such, an undesirable driving condition can result if a steering mode change occurs while the vehicle is travelling at a relatively high speed.
U.S Pat. No. 4,671,523 shows another vehicle steering system including a front set of steerable wheels and a rear set of steerable wheels. The rear set of steerable wheels includes an actuatable locking pin adapted to, in a first position, permit steerable motion of the rear set of steerable wheels and, in a second position, locks the rear set of steerable wheels in a straight-ahead direction. The pressure. The fluid pressure is functionally related to applied steering torque. The '523 patent states that ". . . steering torque is affected by a number of driving specific parameters, such as the speed of the vehicle, the friction of the road surface, the curve radius, etc." The '523 patent further states that ". . . steering of the rear wheels is eliminated by very small steering torques. Thereby, unproblematical tracking of the vehicle is ensured at high speeds or in long curves or during slightly changing steering maneuvers."